A rigid floor covering can consist of wood, timber based materials and/or plastics. Among other things, laminate floors are known which are composed of individual panels and are laid as a floating floor. A single panel consists for example of an HDF-support sheet and a laminated layer applied thereon, which is responsible for the appearance of the floor among other things. In the case of parquet panels the décor is build by a layer of wood. Above the decor usually a transparent protective layer is provided. Underneath a supporting plate usually a further layer is provided, which provides a counter-act. In case of a laminate layer the counter acting or balancing layer is usually formed of a paper. In the case of the parquet panel it is usually a layer of wood.
If people move about in the room a noise is created, which is called footstep sound. The footstep sound is clearly greater in the case of a floor covering build from rigid floor panels as it is the case with rooms, which are provided with carpets or elastic floor coverings such as PVC. The noise development, i.e. the footstep sound, is based on reflection of shock waves introduced into the floor when it is walked on. The amplitude spectrum of the shock of sound waves depends on the room to floor, floor to substrate boundaries and on the attenuation in the different layers. The noise development is particularly high if a layer of air remains between two layers, i.e. for example between the laminated floor and the screed below it.
In order to reduce the noise development from walking, the footstep sound may be absorbed by converting it into heat and thus reduce respectively absorbing it. Another possibility is to shift the frequencies. Depending on the frequency a noise may be perceived as pleasant or unpleasant. By shifting the frequencies it may be achieved, that the footstep sound is perceived as more pleasant and is thus improved.
To absorb or to improve the footstep sound various mat-like materials, such as closed-cell polyethylene foam, cork, polymer bounded mats of recycled rubber and cork, corrugated card or soft wood fiber fabrics are used as an under layer under a rigid floor covering above the screed, as it is known from WO 01/09461. The sound absorbing effect that can be achieved by this method is, however, unsatisfactory. Attempts have therefore been made to stick the above mat-like materials directly to the back of a rigid floor covering, i.e. on the base of a floor panel, for example. Disadvantageously this involves high technical complexity. The costs are consequently high. Overall, the sound reduction is unsatisfactory in relation to technical complexity.
From the document DE 196 20 987 C1, for example, an insulating film is known, which is equipped with an adhesive strip. It is intended to stick the insulating film onto the bottom of a rigid floor covering so as to reduce noise development when a floor is walked on.
From document DE 43 29 766 A1 it is known to provide a polymer material for the footstep sound insulation of a floor. According to the document DE 38 35 638 A1 an insulating mat of expandable polystyrene is used as an insulation in rigid floor coverings. From the document WO 01/09461 A1 it is known to attach a layer of thermoplastic material fixedly with rigid panels of a floor covering, in order to achieve good absorption properties. The WO 93/24295 discloses the application of layers of plastic material with a thin wood covering, which is elastic and thus not rigid in the sense of the present invention.
In the prior art usually a disadvantageous relatively thick sound absorption layer is required, to achieve the desired effect. However, the installation of thick floor coverings is usually problematic in buildings, since due to doors and floors in adjacent rooms the laminate floor may not be too thick. Otherwise, steps are created to adjacent rooms or a door cannot be opened or closed any longer. Steps are undesirable for optical reasons. Additionally, they are so-called trip hazards. Doors have to be adjusted to the different height of the floor covering.
According to document WO 02/100638 A1 it is suggested to provide next to a very thin layer of thermoplastic material with a marked physical relaxation behaviour at ambient temperature a final paper layer on the bottom side of a panel. This teaching is based on the idea to convert sound inside of the thermoplastic layer into heat and to this aim direct sound by means of a fixed connection between the panel and the thermoplastic layer particularly well into the thermoplastic layer.